1. Field of the Invention
This invention relates to a formed article of an amorphous alloy having a hardened surface and a method for the production thereof. More particularly, this invention relates to a surface hardening technique for forming a ceramic hard layer on the surface of a part or article, particularly component parts of an optical connector such as capillary, ferrule, sleeve, and V-grooved substrate and component parts of a golf club head such as face, crown, and sole.
2. Description of the Prior Art
A formed article made of a metallic material has an advantage that it requires no machining or easy machining in contrast with a formed article made of a ceramic material. Among other metallic materials, an amorphous alloy (metallic glass) has such advantages that it permits faithful reproduction of the shape and size of a cavity of a metal mold by the metal mold casting from melt thereof or by the molding process utilizing the viscous flow of the material resorting to the glass transition region thereof and allows manufacture of a formed article with high accuracy at a low cost, without requiring further machining. Furthermore, since the amorphous alloy is excellent in mechanical strength and chemical properties, it is applied to various parts and articles. For instance, Japanese Patent Application, KOKAI (Early Publication) No. (hereinafter referred to briefly as xe2x80x9cJP-A-xe2x80x9d) 10-186176 and JP-A-10-311923 disclose components parts of an optical connector made of an amorphous alloy, such as ferrule, capillary, and sleeve, and JP-A-11-104281 discloses a golf club head in which at least a face part is made of an amorphous alloy.
Such articles as an optical connector which is prone to frequent attachment and detachment of the counter connector and a golf club head which is prone to frequent collision against another object (a golf ball or other objects) inevitably require to possess high resistance to abrasion. Heretofore, however, no example of subjecting the article made of an amorphous alloy to any specific coating or surface hardening is known in the art. This is because that the amorphous alloy is considered to possess higher strength and higher hardness as compared with a general-purpose metal and further that it is difficult to effect the hardening treatment thereof by the normal surface hardening process. When the amorphous alloy is subjected to an abrasion test using the counter material of ceramics, however, there is the possibility of producing an abrasive powder due to the difference in hardness because the amorphous alloy has the hardness of about 500 Hv, while the ceramics have the hardness of about 1200 Hv. It is therefore desired to further improve the abrasion resistance of the component parts of the optical connector which is predominantly manufactured from the ceramic material such as zirconia and which is prone to frequent attachment and detachment of the counter connector and a golf club head which is prone to frequent collision against pebblestones or the like upon making a shot.
As a surface hardening method, the method of coating a hard film of TiC, TiN, etc. on the surface of a substrate by the sputtering process or by the ion plating process is commonly known in the art. However, such methods pose a problem of the separation of the hard film due to the difference in the coefficient of thermal expansion between the base material and the hard film, depending on the film thickness. Further, such methods require the expensive apparatus such as the sputtering device or the ion plating device. Moreover, they require a further treatment such as the coating of an intermediate film to obviate the difference in the thermal expansion coefficient. However, this treatment cannot be applied to an amorphous alloy because the higher temperature of treatment causes the crystallization of the amorphous alloy.
On the other hand, the surface of the optical connector is hardened by an ion exchange treatment as effected in the article of crystallized glass. This method also has the similar problem as mentioned above that an apparatus for the ion exchange treatment is expensive.
A steel material is commonly subjected to carburizing, nitriding, flame hardening or the like to harden the surface thereof. Such surface hardening treatments require a special expensive apparatus. Furthermore, such methods cannot be applied to an amorphous alloy because the treating temperature is so high as to cause the crystallization of the amorphous alloy.
It is, therefore, a fundamental object of the present invention to provide a formed article of an amorphous alloy having significantly improved resistance to abrasion besides the excellent properties naturally possessed by the amorphous alloy and a surface hardening technique therefor.
A further particular object of the present invention is to provide a treating method which allows hardening of the surface part only of a formed article at a lower cost as compared with a conventional surface hardening process, without changing the amorphous structure of the matrix material and the size of the article.
To accomplish the object mentioned above, according to the present invention, there is provided a method for the production of a formed article of an amorphous alloy having a hardened surface, which method comprises subjecting a formed article of a substantially amorphous alloy containing an amorphous phase in a volumetric ratio of at least 50% to a heat treatment in an atmosphere containing a reactive gas under the conditions of temperature and time falling within the amorphous region in the isothermal transformation curve (TTT curve) of the material, thereby forming a ceramic hard layer on the surface of the article.
Preferably the formed article mentioned above is made of an amorphous alloy possessing a glass transition region, particularly a glass transition region of a temperature width of not less than 30 K.
In a more concrete preferred embodiment, the heat treatment mentioned above is carried out in an atmosphere containing oxygen and/or nitrogen at a concentration of not less than 1 ppm or in the air and at a temperature of not less than the lowest temperature required for the oxidation or nitriding reaction of at least one component element of the matrix material. More preferably, the heat treatment mentioned above is carried out under the conditions falling within the range enclosed by the following four points; (1) treating temperature of 350xc2x0 C.xe2x80x94treating time of 10 minutes, (2) treating temperature of 350xc2x0 C.xe2x80x94treating time of 120 minutes, (3) treating temperature of 420xc2x0 C.xe2x80x94treating time of 120 minutes, and (4) treating temperature of 450xc2x0 C.xe2x80x94treating time of 10 minutes.
By carrying out the above heat treatment, it is possible to produce a hard layer containing an oxide and/or nitride of at least one component element of the matrix material in such a manner that the content of oxide and/or nitride gradually decreases in the depth direction from the surface toward the inside of the matrix material, with an amount of change in the surface roughness or size of not more than 10 xcexcm.
The present invention further provides a formed article of an amorphous alloy subjected to the aforementioned surface hardening treatment, characterized by having a ceramic hard layer containing a ceramic component formed by the transformation of at least one component element of the matrix material into ceramic in the surface of the formed article of a substantially amorphous alloy containing an amorphous phase in a volumetric ratio of at least 50%.
In a more concrete preferred embodiment, the ceramic hard layer mentioned above contains a ceramic component comprising an oxide and/or nitride of at least one component element of the matrix material and has such a gradient structure that the content of ceramic component is continuously or stepwise increased in the direction toward the surface.
The formed articles of an amorphous alloy as mentioned above may be useful as parts or articles in various fields. Since the formed articles of an amorphous alloy have excellent resistance to abrasion besides good mechanical strength and chemical properties, they are befit for component parts of an optical connector such as capillaries, ferrules, sleeves, and V-grooved substrates and component parts of a golf club head such as faces, crowns, and soles.